In a wireless cellular network system, each cell typically includes a base station in communication with a user equipment. Types of user equipments include a mobile phone, a notebook, a PDA, etc. Before a data transmission process is started, the base station will transmit a reference signal (e.g., a pilot signal) to the user equipment, and the user equipment will derive a channel estimation value from the reference signal. The reference signal is a known sequence of signals transmitted at a specific time instant and at a specific frequency as prescribed, and the quality of channel estimation will be influenced by interference, noise and other factors.
Typically the user equipment is located at different geographical locations where there may be different received signal strengths as well as noise and interference strengths. Thus some user equipment can communicate at a higher rate, e.g., a user equipment located at the center of the cell, and some other user equipments can communicate only at a lower rate, e.g., a user equipment located at the edge of the cell. Data is transmitted to the user equipment preferably in a format matching a channel condition of the user equipment in order to make full use of a transmission bandwidth of the user equipment. A technology to match a format in which data is transmitted to the user equipment with a channel condition of the user equipment is referred to as link adaptation.
In a system with use of the technology of Orthogonal Frequency Division Multiplexing (OFDM), a plurality of OFDM symbols can he transmitted concurrently over different sub-carriers. The frequency spacing between the sub-carriers can exactly ensure them to be orthogonal to each other. An OFDM modulator converts an input data symbol stream into a plurality of parallel data symbol streams through serial-to-parallel conversion. The sub-carriers on both sides of a bandwidth are precluded from data transmission and referred to as guard bandwidths. Data symbols over some sub-carriers in the range of the data bandwidth will be designed as symbols known to a receiver, and the symbols over these sub-carriers are referred to as pilot symbols from which the receiver can estimate channel information for the purpose of coherent demodulation.
The technology of Orthogonal Frequency Division Multiple Access (OFDMA) is an OFDM-based multiple access transmission technology. A frequency resource in a system bandwidth is divided into resource blocks of a specific size, and each resource block is the smallest resource unit of resource allocation in the frequency domain. The OFDMA system schedules different user equipments onto different resource blocks in the range of the system bandwidth for the purpose of orthogonal transmission between users.
The OFDMA technology is adopted in the downlink by the 3rd Generation Partnership Project (3GPP) for Long Term Evolution (LTE), where an OFDM symbol is the smallest granularity of a resource per sub-frame (1 ms) in the time domain, and each sub-frame includes 12 or 14 OFDM symbols. A sub-carrier is the smallest granularity of in the frequency domain. The smallest time-frequency unit is defined as an elementary resource unit, i.e., a Resource Element (RE). A physical Resource Block (PRB) is defined as the smallest resource allocation unit in an LTE system. A PRB includes REs corresponding to 12 consecutive sub-carriers over all of OFDM symbols in a sub-frame. A user equipment may be scheduled onto consecutive or nonconsecutive physical resource blocks.
In order to assist the base station in link adaptation, the user equipment needs to report a Channel Quality Indicator (CQI) according to a channel condition thereof. The CQI reported by the user equipment corresponds to a specific time-frequency resource. The CQI represents a transmission capacity over the time-frequency resource. The CQI needs to be calculated by the user equipment measuring undergone interference I and noise power N0. For example, a straightforward CQI calculation formula is:
                              CQI          =                      Q            ⁡                          (                              P                                  I                  +                                      N                    0                                                              )                                      ,                            Formula        ⁢                                  ⁢        1            
Where P is received signal power of the user equipment, and Q(·) is a quantization function; and in practice, the user equipment may measure the sum of I+N0.
In the prior art, the technology of coordinated multipoint transmission refers to coordination between a plurality of geographically separated transmission points. Typically the plurality of transmission points refer to base stations of different cells or a plurality of base stations in the same cell. The technology of coordinated multipoint transmission is categorized into downlink coordinated transmission and uplink joint reception. A solution to the technology of downlink coordinated multipoint transmission is primarily categorized into coordinated scheduling and joint transmission.
Coordinated scheduling refers to that the respective base stations coordinate time, frequency and space resources between the cells to allocate mutually orthogonal resources for different User Equipments (UEs) to thereby avoid mutual interference. Inter-cell interference is a predominant factor restricting the performance of a UE at the edge of a cell, and inter-cell interference can be lowered through coordinated scheduling to thereby improve the performance of the UE at the edge of a cell. Referring to FIG. 1, for example, coordinated scheduling of three cells can schedule three UEs possibly interfering with each other onto mutually orthogonal resources to thereby effectively avoid interference between the cells.
Joint transmission refers to concurrent transmission of data in a plurality of cells to a UE to thereby enhance a received signal of the UE. Referring to FIG. 2, for example, data is transmitted in three cells to the same UE over the same resource, and the UE receives signals of the plurality of cells concurrently. On one hand, superposition of the useful signals from the plurality of cells can improve the quality of the signals received by the UE. On the other hand, interference to which the UE is subjected can he lowered to thereby improve the performance of the system.
In order to effectively support coordinated multipoint transmission, the user equipment also needs to estimate channel state information of a coordinating cell base station to the user equipment in addition to a serving cell. Taking a Long Term Evolution-Advanced (LTE-A) system as an example, channel state information is estimated in the LTE-A system by measuring a pilot. For example, an assumed mapping relationship between measurement pilots and data in a PRB is as illustrated in FIG. 3, where the first two OFDM symbols are configured to transmit control information, and a data region starts from the third OFDM symbol. The data region includes REs for transmitting measurement pilots (simply pilot REs) and REs for transmitting data (simply data REs, i.e., PDSCH REs illustrated in FIG. 3). In a practical application, measurement pilots used by adjacent cells are typically mapped onto different REs because the pilot REs typically have high power and are transmitted throughout the bandwidth and there is strong interference between the measurement pilots mapped onto the same RE, thus degrading the precision of channel estimation. As illustrated in FIG. 3, for example, in a cell 1, the user equipment needs to perform channel estimation on REs corresponding to measurement pilots in a cell 2 and a cell 3 to thereby obtain channel state information of the cell 2 and the cell 3, and on these REs, downlink data transmission may possibly be scheduled in the cell 1, for example, data may be transmitted over a Physical Downlink Shared Channel (PDSCH), and as such the measurement pilots transmitted in the cell 2 and the cell 3 may be subjected to interference from data transmission in the cell 1. That is, although the user equipment is located in the cell 1, the data transmitted in the cell 1 may still be inference to the user equipment estimating channels of the cell 2 and the cell 3, and thus for the user equipment in the cell 1, the strength of its received signal of the cell 1 is typically far above the strengths of signals of the cell 2 and the cell 3 so that the user equipment in the cell 1 obtains the measurement pilots of the cell 2 and the cell 3 at a low Signal to Interference and Noise Ratio (SINR) and can not obtain satisfactory precision of channel estimation.
In view of the foregoing problem, in the prior art, the REs on which the cell 2 and the cell 3 transmit the measurement pilots can be left blank, that is, have null data transmitted, in the cell 1, and this solution is referred to RE muting. Referring to FIG. 4, for example, the REs on which the cell 2 and the cell 3 transmit the measurement pilots are set to REs for muting (simply MUTING REs) in the cell 1.
Since the user equipment needs to estimate interference to the serving cell from an adjacent cell to thereby feed back channel state information (generally a CQI), and in the RE muting solution, interference, from the adjacent cell, at the locations of the measurement pilots in the serving cell has been cancelled (for example, no data is transmitted in the cell 2 and the cell 3 in the REs on which the measurement pilots are transmitted in the cell 1), interference calculated by the user equipment at the locations of the measurement pilots in the serving cell is far below actually undergone interference. Referring to FIG. 5 and FIG. 6, for example, it is assumed that three cells as illustrated in FIG. 5 are configured with MUTING REs, and measurement pilots of the three cells are structured as illustrated in FIG. 6. The cell 1 has those REs left blank which “collide” with the measurement pilots of the cell 2 and the cell 3 and on which no data is transmitted. The user equipment in the cell 1 can estimate channel status of the cell 2 and the cell 3 on the blank REs to thereby avoid interference from the cell 1, so the precision of channel estimation by the user equipment for the cell 2 and the cell 3 can be thus improved. This is beneficial to Coordinated Multipoint (CoMP) transmission because channel state information of the adjacent cell to the user equipment needs to be obtained for CoMP transmission.
However in the prior art, the UE may perform channel estimation with respect to an interference source variable with a changing application scenario, including but not limited to the following several situations:
For example, in the situation of joint transmission, all of cells in a measurement set may transmit useful signals to the UE, and at this time, interference to the UE may originate only from other than the measurement set, and the UE shall calculate and feed back channel state information only from interference of cells other than the measurement set.
In another example, due to the use of a dedicated demodulation pilot, the UE can have its transmission scheme switched dynamically between single cell transmission and CoMP transmission, and as a result of actual scheduling, some UE configured in the CoMP mode may need to perform single cell transmission and thus has to report a CQI of single cell transmission. At this time, interference to the UE may originate from other than the serving cell.
In still another example, in some CoMP transmission schemes, a coordinating cell with strong interference to the UE may avoid interference to the UE by keeping silent. The silence of the coordinating cell may have the level of interference to the UE significantly changed to thereby degrade the precision of CQI estimation, and the UE reporting a CQI can not determine whether the coordinating cell is silent. Thus a conservative practice is to have the UE calculate and report the CQI respectively for both interference situations.
In view of this, due to the introduction of the CoMP technology, the UE may perform channel estimation and further calculate CQIs possibly with respect to different interference sources in different application scenarios, and consequently the existing RE configuration solution may have to calculate interference with respect to the different interference sources, so that the UE has to estimate interference respectively for these interference sources. The invention proposes a solution to this demand.